Integrated Transcriptomic and Metabolomic Analyses Shed Light on the Regulation of Aromatic Amino Acid Biosynthesis in a Novel Albino Tea (Camellia sinensis) Mutation

Off-white or yellowish shoots are common in tea plants (Camellia sinensis L.), and such albino variations are often accompanied by metabolic reprogramming, including increased contents of amino acids and lower levels of polyphenols. Nonetheless, the molecular mechanisms that underlie these albino variations remain to be fully clarified. Here, we examined the ultrastructural characteristics of novel, naturally occurring, yellowish mutated tea leaves and performed metabolomic analyses on green and albino leaves and stems. Then, transcriptomic analyses were also conducted on green and albino leaves to investigate the mechanistic basis of the albino variation. As expected, the cells of albino tea leaves contained fewer and smaller chloroplasts with disorganized thylakoids and smaller starch granules. Widely targeted metabolomics analysis revealed 561 differentially abundant metabolites between green and albino leaves and stems, but there was little difference between green and albino stems. Then, RNA sequencing of green and albino leaves revealed downregulation of genes associated with light harvesting and photosynthesis, and integration of the metabolomic and transcriptomic results indicated that biosynthesis of aromatic amino acids (AAAs) was strongly upregulated in albino leaves. To gain additional insight into the molecular basis of the increased AAA levels, Oxford Nanopore long-read sequencing was performed on green and albino leaves, which enabled us to identify differences in long non-coding RNAs (lncRNAs) and alternatively spliced transcripts between green and albino leaves. Interestingly, the amino acid biosynthesis genes arogenate dehydratase/prephenate dehydratase (ADT) and serine hydroxymethyltransferase (SHMT) were highlighted in the lncRNA and alternative splicing analyses, and the transcription factor genes PLATZ, B3 Os04g0386900, and LRR RLK At1g56140 showed significant changes in both expression and alternative splicing in albino leaves. Together, our data suggest that biosynthesis of AAAs might be crucial for albino mutations in tea plants and could be coordinated with the regulation of lncRNAs and alternative splicing. This is a complex regulatory network, and further exploration of the extensive metabolic reprogramming of albino tea leaves will be beneficial.